It is known that aromatic isocyanates may be converted into primary amines by acid hydrolysis. However, the reaction is by no means complete because the amine formed during hydrolysis reacts further with unreacted isocyanate to form the corresponding urea. This secondary reaction cannot be suppressed even by using excess strong mineral acid. A fairly recent example may be found in Japanese Pat. No. 55007-827.
German Auslegeschrift No. 1,270,046 describes a process for the production of certain primary aromatic amines containing polyalkylene glycol ether segments in which reaction products of aromatic diisocyanates or triisocyanates with polyalkylene glycol ethers and/or polyalkylene glycol thioethers, preferably those having molecular weights in the range of from 400 to 4000, are reacted with secondary or tertiary carbinols, followed by thermal decomposition in an inert solvent (optionally in the presence of acid catalysts). The disadvantage of this process is that thermal decomposition of the urethanes is accompanied by the formation of flammable, readily volatile alkenes which are explosive in admixture with air so that appropriate precautionary measures have to be taken.
German Auslegeschrift No. 1,694,152 relates to the production of prepolymers containing at least two terminal amino groups by reacting hydrazine, aminophenyl ethyl amine or other diamines with an NCO-prepolymer of a polyether polyol and polyisocyanate (in an NCO:NH ratio of 1:1.5 to 1:5). In this process, unreacted amine has to be carefully removed in another step because it catalyzes the reaction with polyisocyanates to a considerable extent, thus leading to short processing times, and it actually becomes a reactant itself.
Another possible method for synthesizing polyamines containing urethane groups is described in French Pat. No. 1,415,317. NCO-prepolymers containing urethane groups are converted by treatment with formic acid into the N-formyl derivatives which are hydrolyzed to form terminal aromatic amines. The reaction of NCO-prepolymers with sulfamic acid in accordance with German Auslegeschrift No. 1,155,907 also leads to compounds containing terminal amino groups. In addition, relatively high molecular weight preadducts containing aliphatic, secondary and primary amino groups are obtained in accordance with German Auslegeschrift No. 1,215,373 by reacting relatively high molecular weight hydroxyl compounds with ammonia under pressure at elevated temperatures and in the presence of catalysts. These preadducts may also be obtained in accordance with U.S. Pat. No. 3,044,989, by reacting relatively high molecular weight polyhydroxyl compounds with acrylonitrile, followed by catalytic hydrogenation. According to German Offenlegungsschrift No. 2,546,536 and U.S. Pat. No. 3,865,791, relatively high molecular weight compounds containing terminal amino groups and urethane groups are also obtained by reacting NCO-prepolymers with enamines, aldimines or ketimines containing hydroxyl groups, followed by hydrolysis.
It has surprisingly been found that certain primary aromatic or aliphatic polyamines containing urethane and/or urea and/or biuret and/or isocyanurate groups and, optionally, alkylene ether and/or carbonate and/or acetal groups may be directly obtained by mixing compounds containing NCO-groups, preferably NCO-pre-adducts, with selected bases (preferably aqueous base solutions in an equivalent or excess quantity, based on the NCO-content) and treating the corresponding carbamate salt with a strong acid, preferably a mineral acid, in a quantity equivalent to the quantity of base used.
German Offenlegungsschrift No. 2,948,419 describes a process for the production of polyamines, containing urethane and/or urea groups by hydrolyzing compounds containing terminal isocyanate groups, in which the carbamate is prepared using alkali and subsequently converted into a salt by adding an excess quantity of mineral acid, accompanied by the elimination of CO.sub.2. Excess mineral acid is then neutralized in a further step by the addition of a base and the polyamine thus released is separated off from the reaction product by known methods.
The invention provides for an improvement in this process, saving a working-up stage and reducing the salt load. By virtue of this improvement, the carbon dioxide is eliminated much more quickly and completely from the carbamate. The process according to the invention is particularly suitable for continuous working.